1. Field of the Invention
The present invention relates to an electronic equipment having a microprocessor and a memory, and more particularly to an electronic equipment suitable for use as a laser beam printer, a facsimile machine and so on.
2. Description of the Prior Art
The development of electronic technology in recent years has been remarkable. Since an IC device is highly integrated, high performance electronic equipment such as a personal computer, a word processor, an office computer, a facsimile machine and a laser beam printer can be attained with a small size and a low cost.
On the other hand, as electronic technology develops, electronic equipment is used in various fields and the manners in which it may be used are versatile. As a result, a single type of electronic equipment is required to have various specifications, and performances depending on the manner of its use. If an equipment which meets all of those requirements is manufactured and furnished, it may be an overperformance product to a certain user and hence a low cost-performance product.
In order to resolve the above problem, the following solution is usually used. The main unit of electronic equipment has only the essential basic functions, and other auxiliary electronic equipment, such as an expansion memory or an expansion character generator, is connected thereto depending on the user specification. It is called a ROM cartridge or RAM cartridge and used to expand the function of the main unit of electronic equipment, expand types of character fonts or expand a RAM which temporarily stores data.
FIG. 1 shows a functional configuration of a prior art electronic equipment. It mainly shows a memory access function. Numeral 1 denotes a main unit of an electronic equipment such as a personal computer, which includes a central processing unit (CPU) 101 for controlling the electronic equipment, and an internal memory 102 including a ROM for storing a control program and a RAM. Numeral 103 denotes a wait signal generator which generates a wait signal on a line 104 to temporarily suppress memory accessing by the CPU 101. Usually, when the CPU reads or writes into the memory, the CPU is caused to wait for a predetermined time period for each memory accessing in order to match a memory access time (memory cycle) to a CPU control speed so that the CPU retrieves a data after the read data has become fully effective or the CPU produces a write pulse to write a data into the memory after the data from a bus has become fully writable into the memory. The wait signal generator 103 is provided to establish such a wait time. When the CPU 101 accesses the internal memory 102, the wait signal generator 103 causes the CPU 101 to wait for a predetermined time period depending on a rated access time of the memory used. Numeral 2 denotes an auxiliary memory corresponding to the expansion ROM cartridge or RAM cartridge described above, and includes an external memory 201 comprising a ROM or RAM. Usually, the external memory 201 uses memory elements having the same access time as the internal memory 102, and the wait signal generator 103 causes the CPU 101 to wait for the same predetermined time period when the CPU 101 accesses the external memory 201, as it does when the CPU 101 accesses the internal memory 102. Thus, by coupling the auxiliary memory 2 to the main unit 1 of the electronic equipment, the overall function of the system is expanded as if the capacity of the internal memory 102 were expanded. However, such a prior art electronic equipment has the following disadvantage.
The external memory 201 must use memory elements which have equal or higher access time to or than that of the internal memory 102. As described above, recent developments in electronic technology have been remarkable and as for the memory elements, the performance thereof have been rapidly improved, such as increase of memory capacity and increase of operation speed, and various types of memory elements from a low priced one to a high priced one are available depending on the performance. On the other hand, in the electronic equipment, it is necessary that various auxiliary memories are available for one main unit of the electronic equipment depending on purpose of processing by a user and a desired price. In other words, it is desirable that optimum memory elements to the user from standpoints of cost and performance can be used. In the prior art electronic equipment, it is necessary that the access time of the external memory 201 of the auxiliary memory 2 is equal to or shorter than the access time of the internal memory 102 of the main unit 1 of the electronic equipment. Accordingly, freedom of selection of the memory elements used in the auxiliary memory 2 is highly restricted both in price and in performance.
In order to resolve the above problem, the wait time for the CPU in the main unit of the electronic equipment may be set sufficiently long so that auxiliary memories having different rated access times can be selectively used. In this case, however, if a auxiliary memory having short access time memory elements is used to attain a high speed operation, an unnecessarily long wait time is required for the external memory and the inherent performance of the auxiliary memory is not accomplished.